Bristol, United Kingdom: Clinical evidence indicating that marijuana use may be casually linked to incidences of schizophrenia or other psychological harms is not compelling, according to a scientific review published online by the journal Addiction.

Investigators at the University of Bristol, Department of Social Medicine assessed the potential health risks of cannabis, particularly whether use of the drug may be causally linked with mental illness.

Authors wrote: "We continue to take the view that the evidence that cannabis use causes schizophrenia is neither very new, nor by normal criteria, particularly compelling. ... For example, our recent modeling suggests that we would need to prevent between 3000 and 5000 cases of heavy cannabis use among young men and women to prevent one case of schizophrenia, and that four or five times more young people would need to avoid light cannabis use to prevent a single schizophrenia case. ... We conclude that the strongest evidence of a possible causal relation between cannabis use and schizophrenia emerged more than 20 years ago and that the strength of more recent evidence may have been overstated."

In 2007, an analysis in the British medical journal The Lancet estimated that experimenting with marijuana could increase one's risk of developing a psychotic illness later in life by some 40 percent. Following this report, Parliament in 2008 voted to reclassify marijuana as a Class B substance, making its possession punishable by up to five years in prison.

University of Bristol researchers also criticized Parliament's reclassification of the drug, which took effect earlier this year. They concluded: "The only important possible benefit of prohibition is prevention of cannabis use. There is little or no evidence that it effectively achieves this benefit. Patterns of cannabis use in the population appear to be independent of the policy surrounding use, and criminalizing individual cannabis users does not appear to modify their use in a healthy way."

Overall, investigators determined that marijuana's most significant health risk was its association and reinforcement with tobacco smoking.

For more information, please contact Paul Armentano, NORML Deputy Director, at: paul@norml.org. Full text of the review, "How ideology shapes the evidence and the policy: what do we know about cannabis use and what should we do," appears online in the journal Addiction.